Abstract
The MNiSn (M = Ti, Zr, Hf) n-type semiconductor half-Heusler alloys are leading candidates for the use as highly efficient waste heat recovery devices at elevated temperatures. For practical applications, it is crucial to consider also the environmental stability of the alloys at working conditions, and therefore it is required to characterize and understand their oxidation behavior. This work is focused on studying the surface composition and the initial oxidation of HfNiSn alloy by oxygen and water vapor at room temperature and at 1000 K by utilizing X-ray photoelectron spectroscopy. During heating in vacuum, Sn segregated to the surface, creating a sub-nanometer overlayer. Exposing the surface to both oxygen and water vapor resulted mainly in Hf oxidation to HfO2 and only minor oxidation of Sn, in accordance with the oxide formation enthalpy of the components. The alloy was more susceptible to oxidation by water vapor compared to oxygen. Long exposure of HfNiSn and ZrNiSn samples to moderate water vapor pressure and temperature, during system bakeout, resulted also in a formation of a thin SnO2 overlayer. Some comparison to the oxidation of TiNiSn and ZrNiSn, previously reported, is given.
Highlights
Comparing the results obtained here to the results found during previous reports on Comparing the results obtained here to the results found during previous reports on the other two alloys of the HH family, i.e., TiNiSn [10] and ZrNiSn [11], can give an insight the other two alloys of the HH family, i.e., TiNiSn [10] and ZrNiSn [11], can give an insight
Comparing the results obtained here to the results found during previous reports on the other two alloys of the HH family, i.e., TiNiSn [10] and ZrNiSn [11], can give an insight into the surface composition and the initial oxidation mechanism
It should be noted that since usually there is a similarity between segregation to free surfaces and to grain boundaries, we should expect some enrichment of Sn at the grain boundaries, which may affect the transport behavior across the sample or a device
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Kang et al [9] demonstrated that MNiSn HH alloys can have relatively high oxidation resistivity in air at the operation temperature, and they correlated this oxidation resistivity to the formation of Ni–Sn intermetallic protective layers. Their data at 873 K and after 72 h indicate the formation of an oxide layer, i.e., a few tens micrometer thick, with most of the oxide phases found to be SnO2 , HfO2 and ZrO2. Temperature effects on the surface composition and the initial oxidation of HfNiSn alloy by oxygen and water vapor at room temperature (RT) and 1000 K were studied, utilizing X-ray photoelectron spectroscopy (XPS). Understanding the mechanisms that govern the surface composition change and oxidation of these relatively simple ternary alloys, that are presented here, can serve as building blocks to understand the oxidation behavior of more complex HH alloys
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
